At a glance
- Adult stage
- Lives 7 to 10 days, mostly a nuisance
- Larval habitat
- Top 2 to 3 inches of moist growing mix
- First correction
- Let the top 1 to 2 inches dry between waterings
- One generation
- About 17 days at 75°F, or 3 to 4 weeks at 65 to 75°F
- Larval controls
- Bti, S. feltiae nematodes, or a neem product labeled for soil drench
- High-risk plants
- Seedlings and young plants
Larvae mainly eat fungi and decaying organic matter, but in large numbers they can damage roots, especially on seedlings and young plants. Keep an infested plant where it is, because moving it lets the flies emerge near other pots. The sections below cover confirming the pest, fixing the mix, trapping adults, treating larvae and keeping the gnats from returning.
How do you confirm you have fungus gnats?
Adult fungus gnats are small, black, mosquito-like flies about 1/8 inch long, with long legs and antennae and a Y-shaped pattern on the forewings. They are weak fliers that move in short, erratic patterns near the infested plant, and they often gather around window frames.
The larvae do the damage. They are wormlike, translucent to whitish and legless, with a shiny black head, and they live mostly in the top 2 to 3 inches of the mix, where they eat fungi, algae and decaying plant matter. Where the mix is very wet and gnats are abundant, larvae can leave slime trails on the surface that look like those of small snails or slugs.
To confirm larvae, push a 1/4-inch slice of raw potato into the mix with the cut side down, leave it a few days, and turn it over. Larvae move to the potato and start feeding within a few days, so a larva with a black head on the underside supports the identification. Remove infested slices and replace them with fresh ones.
Other small flies get mistaken for fungus gnats, so compare yours with these before you treat.
| Fly | Where it breeds | How to tell it from a fungus gnat |
|---|---|---|
| Shore fly | Algae and decomposing organic matter | Stouter, with short bristle-like antennae instead of long ones |
| Moth fly | Stagnant water rich in organic matter | Broad, feathery wings that look like a tiny moth's |
| Whitefly | Leaf undersides | White, about 1/16 inch long, and flutters up from the foliage when the plant is disturbed |
Why do fungus gnats keep coming back?
A female lives only seven to 10 days, but she may lay up to 200 eggs in the cracks and crevices of the mix, and mix with a lot of peat moss is especially attractive to her. One generation, from one female to the next, takes about 17 days at 75°F and three to four weeks at 65 to 75°F.
- Eggs hatchIn about 3 days, in damp organic mix.
- Larvae feedFor about 10 days in the mix, then they develop into pupae.
- Pupae developAdults emerge about 4 days later.
- Adults lay the next eggsA new generation starts, and the warmer the room, the faster it runs.
Generations overlap, so flies keep emerging from pupae in the mix after the adults you can see are gone.
Gnats are most noticeable in late fall and winter. Houseplants kept outdoors in summer are commonly colonized, and the gnat populations grow in warm indoor temperatures once those plants come inside. Mix that ages and breaks down holds more water, and shorter, cooler days slow plant growth and water use, so a pot watered on the same schedule as in summer stays damp.
How do you make the potting mix less inviting?
The most important step is letting the top 1 to 2 inches of mix dry between waterings. Dry mix lowers the survival of eggs and newly hatched larvae, and it makes the pot less attractive to egg-laying females. Dry it as far as the plant allows without wilting, and use the houseplant watering guide to judge when each plant needs water.
Gnats also breed in decaying material, so the mix and the pot need cleaning up too.
Clear out the breeding sites
- Empty saucers and clean up any standing water
- Confirm every pot has open drainage holes
- Remove decayed bulbs, roots and other decaying plant matter, which feed the larvae
- Repot mix that has broken down and holds water too long
- Avoid large amounts of manure, blood meal or similar organic fertilizers, and incompletely composted material in the mix unless it is pasteurized
- Cut back watering in winter, when plant growth and water use slow
Repot with fresh mix in a clean pot after washing the old soil off the roots, and never use garden soil for repotting indoor plants. Adding more perlite or sand to the mix improves drainage, and the houseplant soil guide covers mix choices.
A wilting plant that has gnats is not automatically a gnat problem. UC IPM notes that larvae can damage roots enough to cause wilting, but too much or too little water, root decay fungi and poor drainage are much more common causes, so rule those out before blaming the gnats. If you cannot tell whether the plant is too wet or too dry, the overwatering vs underwatering guide helps.
What do yellow sticky cards do, and what do they miss?
Yellow sticky cards catch adults, so they show whether gnats are present and which pots they gather around. Place them under the plant canopy or at the edge of the container, or cut a card into small squares and attach them to skewers pushed into the pots.
Cards cannot control the problem by themselves. Adults live only seven to 10 days and may have laid many eggs before a card catches them, and the larvae and pupae in the mix are untouched. Trapping adult females may still reduce the number of larvae in the next generation, and checking the pots near the busiest cards for wet mix and decaying matter shows where the larvae are feeding.
Which treatments kill fungus gnat larvae?
Change the watering first, and add a larval treatment if the gnats persist several weeks later. Larvae are the target because insecticides aimed at adults do not touch them.
| Option | How it works | What to know |
|---|---|---|
| Bti (Bacillus thuringiensis subspecies israelensis) | A bacterium that is toxic only to fly larvae such as mosquitoes, black flies and fungus gnats | Gives temporary control and does not persist indoors, so it may need repeating at about five-day intervals |
| Steinernema feltiae nematodes | Microscopic roundworms enter the larvae and release a bacterium that digests them, and the larvae die within three to four days | Live and perishable, so they must be applied right away, and they work when temperatures are 60 to 90°F and conditions are moist |
| A neem product labeled as a soil drench | Some neem products can control fungus gnat larvae | Use only a product whose label allows soil use, since a neem spray on the leaves does not reach the larvae |
Mix Bti or nematodes with water and apply them as a soil drench, or spray them onto the mix, following the label. UC IPM says Bti products are readily available at retail nurseries and garden centers, Colorado State says formulations sold for fungus gnat control are generally unavailable at retail outlets, and Minnesota Extension says they are available primarily online, so check what a local store carries before you order.
Insecticides are rarely warranted against these flies in homes. If Bti and nematodes are not available and the gnats are intolerable, pyrethrins or a pyrethroid labeled for houseplants may give temporary control. Spray the surface of the mix, where new adults emerge, and take the plant outdoors if you can, waiting about a day before bringing it back in.
Short-lasting contact products such as pyrethrins, soaps, oils and neem do not give enough long-term control of adults and need repeating every couple of days.
Why is fungus gnat control not working?
Judge the result across a full generation, not after a few days, because pupae in the mix keep producing adults after the visible ones are gone.
| What happens | Likely cause | What to do |
|---|---|---|
| Adults vanish for a week, then return | Sprays and cards reach only adults, while larvae and pupae keep developing in the mix | Treat the larvae and keep drying the top of the mix |
| Gnats persist weeks after you changed watering | The mix may still hold water, because aged mix retains more moisture | Repot into fresh mix in a clean pot and remove decaying matter |
| Bti or nematodes seem to do nothing | Bti may be the wrong Bt subspecies or may not have persisted, and nematodes are live and perishable and need 60 to 90°F and moist mix | Repeat Bti at about five-day intervals, and apply nematodes as soon as they arrive |
| Small flies, but no larvae on the potato | The flies may be shore flies or moth flies | Remove algae and standing water, which are their breeding sites |
How do you keep fungus gnats from coming back?
Gnats are native insects common in lawns and garden soils, so reinfestation from outdoors is very likely, but their numbers can be kept low enough that they stop being a nuisance. The usual ways in are listed below.
- New plants. Fungus gnats are common in newly purchased houseplants. Minnesota Extension suggests keeping a new plant apart for one to two weeks so any pest problems become visible, while Colorado State suggests at least three weeks, a precaution it gives for scale, mealybugs and whiteflies, and inspecting it at the end.
- Plants kept outdoors in summer. They are commonly colonized, so inspect them closely before bringing them back inside.
- Potting mix. Use pasteurized container mix and store it off the ground in closed containers, because incompletely composted organic matter is often infested.
- Windows and doors. Screens keep adults from coming in.
Larvae stunt growth particularly in seedlings and young plants, so start them in fresh pasteurized mix, check purchased starts before they join your trays, and put a yellow card at the edge of the tray early. The seedling watering guide covers moisture checks for seed trays.
Pro Tips
- Let the top 1 to 2 inches of mix dry between waterings, as far as the plant allows.
- Empty saucers so pots do not stand in water.
- Store potting mix off the ground in a closed container.
- Put a yellow sticky card at the edge of the pot or under the plant canopy.
- Repot mix that has broken down and stays wet.
- Keep new plants apart and inspect them before they join the others.
Frequently Asked Questions
Are fungus gnats harmful to plants?
Adults are a nuisance and do not bite. Larvae mostly eat fungi and decaying matter, but in large numbers they can damage roots and stunt growth, particularly in seedlings and young plants.
What kills fungus gnat larvae?
A Bti drench, Steinernema feltiae nematodes, or a neem product labeled for soil drench. Bti does not persist indoors, so it may need repeating at about five-day intervals.
Do sticky traps get rid of fungus gnats?
Not by themselves. They catch adults and show where the gnats are, but adults may lay eggs before they are caught, and the larvae stay in the mix.
Why do I have gnats in winter?
Plants grow and use water more slowly when days are shorter and cooler, so the mix stays damp longer if you keep watering on the same schedule. See how often to water houseplants.
Can raw potato help with fungus gnats?
Yes. Larvae are very attracted to raw potato chunks placed cut side down in the mix, so a chunk shows whether larvae are present and can pull some of them away from the roots. Replace it with a fresh one every few days.
